Plasmonic Non-Dispersive Infrared Gas Sensors

ABSTRACT

Differential, plasmonic, non-dispersive infrared gas sensors are provided. In one aspect, a gas sensor includes: a plasmonic resonance detector including a differential plasmon resonator array that is resonant at different wavelengths of light; and a light source incident on the plasmonic resonance detector. The differential plasmon resonator array can include: at least one first set of plasmonic resonators interwoven with at least one second set of plasmonic resonators, wherein the at least one first set of plasmonic resonators is configured to be resonant with light at a first wavelength, and wherein the at least one second set of plasmonic resonators is configured to be resonant with light at a second wavelength. A method for analyzing a target gas and a method for forming a plasmonic resonance detector are also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/918,638 filed on Mar. 12, 2018, the contents of which areincorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to gas sensors, and more particularly, todifferential plasmonic, non-dispersive infrared gas sensors.

BACKGROUND OF THE INVENTION

Most compact carbon dioxide (CO₂) sensors are based on non-(optically)dispersive infrared detection (NDIR). CO₂ has a strong infrared (IR)absorption peak at a wavelength of 4.3 micrometers (μm), which iswell-isolated from those of other gases in the ambient air. In thesimplest type of NDIR spectrometer, an incandescent light source passesthrough a volume of CO₂ gas, through a 4.3 μm filter, and to amid-infrared receiver, which is typically a thermopile. See, forexample, FIG. 1 in Bates et al., “Evaluating Infrared Carbon DioxideSensors for 21′ Century Cell Culture: introducing the Thermo ScientificIR180S1 Infrared CO₂ sensor,” Thermo Scientific, 2014 (downloaded Dec.13, 2017) (5 total pages) (hereafter “Bates”), the contents of which areincorporated by reference as if fully set forth herein. The inferred CO₂concentration is directly proportional to the absorption of the 4.3 μmlight. See Bates.

Drift is the largest source of inaccuracy in NDIR CO₂ sensors. Drift iscaused by a variety of factors, including the incandescent light sourceslowly burning out, the thermopile responsivities drifting, dust anddirt accumulating within the optical path, and the ambient humiditylevel changing. Drift is also intertwined with the size and energy-useprofile of the sensor because, all other things being equal, a moreaccurate CO₂ sensor can use a less intense light source and/or a smalleroptical path length.

To correct for drift, a wavelength slightly removed from the 4.3 μm CO₂absorption peak (such as 4.0 μm) can be used as a reference. One way tomeasure the second wavelength is to use a dispersive filter (see, forexample, FIG. 2 of Bates), which splits the incandescent light intomultiple wavelengths. However, in this configuration, the signal andreference detectors can be subject to one of the detectors selectivelydrifting (e.g., by dust accumulating on one of them).

Thus, improved NDIR gas sensor designs would be desirable.

SUMMARY OF THE INVENTION

The present invention provides differential, plasmonic, non-dispersiveinfrared gas sensors. In one aspect of the invention, a gas sensor isprovided. The gas sensor includes: a plasmonic resonance detectorincluding a differential plasmon resonator array that is resonant atdifferent wavelengths of light; and a light source incident on theplasmonic resonance detector. The differential plasmon resonator arraycan include: at least one first set of plasmonic resonators interwovenwith at least one second set of plasmonic resonators, wherein the atleast one first set of plasmonic resonators is configured to be resonantwith light at a first wavelength, and wherein the at least one secondset of plasmonic resonators is configured to be resonant with light at asecond wavelength.

In another aspect of the invention, a method for analyzing a target gasis provided. The method includes: illuminating the target gas with lightfrom a light source, wherein the light is incident on a plasmonicresonance detector including a differential plasmon resonator arrayhaving at least one first set of plasmonic resonators interwoven with atleast one second set of plasmonic resonators, wherein the at least onefirst set of plasmonic resonators is configured to be resonant with thelight at a first wavelength corresponding to a peak absorptionwavelength of the target gas, and wherein the at least one second set ofplasmonic resonators is configured to be resonant with the light at asecond wavelength corresponding to a reference wavelength; absorbing thelight by the at least one first set of plasmonic resonators and the atleast one second set of plasmonic resonators, wherein the light at thefirst wavelength incident on the at least one first set of plasmonicresonators is dependent on an amount of the light at the firstwavelength absorbed by the target gas; generating a photocurrent signalI₁ in the at least one first set of plasmonic resonators and aphotocurrent signal I₂ in the at least one second set of plasmonicresonators; and determining a concentration of the target gas usingI₁-I₂.

In yet another aspect of the invention, a method for forming a plasmonicresonance detector is provided. The method includes: forming at leastone first set of plasmonic resonators interwoven with at least onesecond set of plasmonic resonators, wherein the at least one first setof plasmonic resonators is configured to be resonant with light at afirst wavelength corresponding to a peak absorption wavelength of thetarget gas, and wherein the at least one second set of plasmonicresonators is configured to be resonant with light at a secondwavelength corresponding to a reference wavelength.

A more complete understanding of the present invention, as well asfurther features and advantages of the present invention, will beobtained by reference to the following detailed description anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an exemplary plasmonic, non-dispersiveinfrared gas sensor according to an embodiment of the present invention;

FIG. 2 is a schematic diagram illustrating plasmons in carbon nanotubesaccording to an embodiment of the present invention;

FIG. 3 is a diagram illustrating an exemplary methodology for processingcarbon nanotubes according to an embodiment of the present invention;

FIG. 4 is a scanning electron micrograph (SEM) image of ribbons ofcarbon nanotubes according to an embodiment of the present invention;

FIG. 5 is a diagram illustrating the absorption spectra from threedifferent ribbons of carbon nanotubes having different lengths accordingto an embodiment of the present invention;

FIG. 6 is a diagram illustrating an exemplary configuration of thepresent plasmonic resonance detector using carbon nanotubes according toan embodiment of the present invention;

FIG. 7 is a diagram illustrating an exemplary gas sensor employing thecarbon nanotube plasmonic resonance detector of FIG. 6 according to anembodiment of the present invention;

FIG. 8 is a diagram illustrating an exemplary methodology for formingthe plasmonic resonance detector of FIG. 6 according to an embodiment ofthe present invention;

FIG. 9 is a diagram illustrating an exemplary methodology for gasdetection using the present plasmonic, non-dispersive infrared gassensors according to an embodiment of the present invention;

FIG. 10 is a diagram illustrating an exemplary plasmonic light sourceaccording to an embodiment of the present invention;

FIG. 11 is a diagram illustrating an exemplary plasmonic, non-dispersiveinfrared gas sensor employing the carbon nanotube plasmonic resonancedetector of FIG. 6 and the plasmonic light source of FIG. 10 accordingto an embodiment of the present invention;

FIG. 12 is a diagram illustrating an exemplary split-ring plasmonicresonance detector according to an embodiment of the present invention;

FIG. 13 is a diagram illustrating an exemplary methodology for formingthe split-ring plasmonic resonance detector of FIG. 12 according to anembodiment of the present invention; and

FIG. 14 is a diagram illustrating an exemplary plasmonic resonancedetector for the detection of multiple target gases according to anembodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Provided herein are gas sensors having wavelength-selective ‘filters’built into the detectors, by using resonant detectors that selectivelyabsorb only certain wavelengths of light. See, for example, gas sensor100 in FIG. 1. Gas sensor 100 includes a light source 102 and aplasmonic resonance detector array 104 both housed within an enclosure101. Enclosure 101 is air-tight except for an air inlet 101 a and an airoutlet 101 b, configured to permit an air sample to enter and exit theenclosure 101, respectively. Air introduced to the enclosure 101 via airinlet 101 a can simply passively diffuse through the enclosure 101, or afan (not shown) can draw it through.

According to one exemplary embodiment, the light source 102 is anincandescent light bulb that is incident on the plasmonic resonancedetector 104. The plasmonic resonance detector 104 includes an array ofplasmon resonators as absorbers. The array includes at least twodifferent types of plasmon resonators. For instance, as shown in FIG. 1the plasmonic resonance detector 104 includes (a first set of)resonators 104 a and (a second set of) resonators 104 b that areresonant at different wavelengths of light. For ease and clarity ofdescription, reference will be made herein to the first resonators 104 abeing resonant at a first wavelength of light and the second resonators104 b being resonant at a second wavelength of light. For instance, byway of example only, the resonators 104 a in plasmonic resonancedetector 104 could be resonant at 4.3 micrometers (μm), the vibrationalresonance of carbon dioxide (CO₂), and the resonators 104 b in plasmonicresonance detector 104 could provide a differential signal by absorbing4.0 μm light which can be used as a reference, e.g., to correct fordrift. Thus, the reference wavelength used is slightly removed from thetarget gas absorption peak. The shift of the reference detector shouldbe spaced away from the signal detector by approximately f/Q, wherein fis the frequency of the vibrational resonance being detected (i.e., 4.3μm for the case of CO₂), and Q is the quality factor of the resonator.For the case of carbon nanotube resonators, Q is approximately 10 (seeChiu et al, Nano Lett)), so 4.0 μm is an appropriate referencefrequency. See, for example, Chiu et al., “Strong and Broadly TunablePlasmon Resonances in Thick Films of Aligned Carbon Nanotubes,” NanoLetters, 2017, 17, pgs. 5641-5645 (August 2017) (hereinafter “Chiu”),the contents of which are incorporated by reference as if fully setforth herein. If the reference frequency is too close to the target gasabsorption peak, the two detectors will experience crosstalk. If it istoo far away, the reference detector may experience different sources ofdrift from the signal detector.

According to one exemplary embodiment, the physical basis of theseresonant absorbers is the excitation of plasmons in carbon nanotubes.These plasmons comprise longitudinal charge oscillations that are boundby the carbon nanotube ends (see FIG. 2) coupled to infrared opticalfields, which can concentrate light into nanoscale volumes. See, forexample, Chiu.

Carbon nanotubes are an exemplary plasmonic material. See, for example,Martin-Moreno et al., “Ultraefficient Coupling of a Quantum Emitter tothe Tunable Guided Plasmons of a Carbon Nanotube,” Physical ReviewLetters, 115, 173601 (2015) (5 total pages), the contents of which areincorporated by reference as if fully set forth herein. Carbon nanotubesare an excellent photothermoelectric material, producing a strongphotocurrent or photovoltage in response to light. Their plasmonresonances are also confined, meaning that the size of a carbon nanotubeplasmon resonator can be much shorter (approximately 50 times smaller)than the free-space wavelength of light that it is detecting. Carbonnanotubes are also highly tunable via the size of the nanotubes involvedin the resonator and the doping level of the nanotubes, allowing them tospan frequencies from the terahertz up to the near infrared.

According to an exemplary embodiment, the carbon nanotubes are processedinto ribbons that are cut to at least two different lengths L1 and L2.See, for example, FIG. 3 which illustrates an exemplary methodology 300for processing carbon nanotubes. Specifically, as shown in step 302carbon nanotubes are aligned horizontally into sheets. In step 304, thesheets are then etched into ribbons, i.e., strips cut/etched from thesheets containing the horizontally-aligned carbon nanotubes cut todifferent lengths. In this example, the ribbons/carbon nanotubes in theribbons are etched to two different lengths, L1 and L2 wherein L1>L2.

FIG. 4 is a scanning electron micrograph (SEM) image 400 of ribbons ofcarbon nanotubes (i.e., the vertical stripes in image 400) each havingaligned carbon nanotubes etched to a length L. Note that the orientationof the carbon nanotubes/ribbons in image 400 is rotated 90° from what isdepicted, for example, in step 304 of FIG. 3.

As highlighted above, these nanotube assemblies function as plasmonresonators.

Etching the carbon nanotubes to different lengths gives them differentresonance wavelengths. See, for example, FIG. 5 which illustrates theabsorption spectra from three different ribbons of carbon nanotubeshaving different lengths. As demonstrated in FIG. 5, this technique oftailoring the resonance via the carbon nanotube length is effective inoverlapping the spectrum of the carbon nanotubes with that of CO₂ (2300cm⁻¹, equivalent to a wavelength of 4.3 mm). In FIG. 5, the lengths ofthe three carbon nanotube plasmon resonators whose attenuation spectrais shown are, from lowest to highest energies, 1 μm, 600 nanometers(nm), and 300 nm.

After they are etched (e.g., into ribbons), the carbon nanotubesfunction as Fabry-Perot resonators for light. The resonance frequenciesof the etched carbon nanotube ribbons (also referred to herein as“nanoribbons”) are proportional to the square root of the free chargedensity on the carbon nanotubes x a thickness of the ribbon (i.e., outof the plane) divided by the carbon nanotube length (L—see FIG. 4,described above). Using this relationship, one skilled in the art giventhe present teachings could tailor the carbon nanotube length to aparticular plasmon resonance frequency for a given gas sensingapplication.

As provided above, in the present gas sensor designs, the conventionalfilter and thermopile is replaced with a plasmonic resonance detector(see, for example, plasmonic resonance detector 104 in FIG. 1) having atleast a first set of plasmonic resonators (e.g., resonators 104 a inFIG. 1) being resonant at a first wavelength of light and at least onesecond set of resonators (e.g., resonators 104 b) being resonant at asecond wavelength of light. For instance, the first set of resonatorscan be resonant at a wavelength corresponding to the absorption peak ofa target gas (such as CO₂), while the second set of resonators isresonant at a different wavelength, i.e., providing a differentialsignal, e.g., to correct for drift. An exemplary configuration of thepresent plasmonic resonance detector using ribbons of carbon nanotubeshaving different lengths as the first/second regions is shownillustrated in FIG. 6. Specifically, plasmonic resonance detector 600shown in FIG. 6 includes (first and second sets of) ribbons 602 a and602 b of carbon nanotubes having at least two different lengths, e.g.,lengths L1 and L2—respectively, whereby the ribbons 602 a of carbonnanotubes having the first length L1 correspond to the first regionsresonant at the first wavelength, and the ribbons 602 b of carbonnanotubes having the second length L2 correspond to the second regionsresonant at the second wavelength.

According to an exemplary embodiment, the first regions are resonant atthe absorption peak of a target gas, and the second regions provide adifferential signal at a slightly different wavelength, or vice versa.For instance, by way of example only, for L1 the plasmon resonance ofthe carbon nanotubes is tuned to be 4.3 μm, to measure the CO₂absorption signal (I_(4.3)), and L2 is tuned to have a resonance at 4.0μm, so as to provide a reference (I_(4.0)). The difference between thetwo signals, differential signal I_(4.3)-I_(4.0), is directlyproportional to the concentration of CO₂ in the optical path (i.e.,between the light source and the carbon nanotube plasmonic resonancedetector array—see for example FIG. 7, described below).

As shown in FIG. 6, (first and second) metal contacts 604 and 606 areformed to the ribbons 602 a and 602 b of carbon nanotubes, respectively.Specifically, the metal contacts 604 are formed contacting the carbonnanotubes (having length L1) in ribbon 602 a, and the metal contacts 606are formed contacting the carbon nanotubes (having length L2) in ribbon602 b. The metal contacts 604 and 606 are used to extract the target gasabsorption signal (I₁) and the reference signal (I₂) from the ribbons602 a and 602 b of carbon nanotubes, respectively. For instance, in thenon-limiting example provided above, the metal contacts 604 are used toextract the CO₂ absorption signal (I_(4.3)), and the metal contacts 606are used to extract the reference signal (I_(4.0)).

According to an exemplary embodiment, one contact to each resonator isgrounded while the other is connected (through wirebonds, chip carriers,etc.—not shown) to a voltage preamplifier, which will convert thephotovoltage to a macroscopic voltage that can be outputted.Specifically, as shown in FIG. 6, metal contacts 604 to carbon nanotubethe ribbons 602 a (i.e., the target gas signal resonators) include afirst contact 604 a which is grounded and a second contact 604 b that isconnected to a voltage preamplifier 608. Likewise, metal contacts 606 tocarbon nanotube the ribbons 602 b (i.e., the reference signalresonators) include a first contact 606 a which is grounded and a secondcontact 606 b that is connected to a voltage preamplifier 610.

The present gas sensor has several advantages over traditional designsthat use separated filters and detectors. First, the spatial pattern ofthe resonant detectors can be interwoven, so that the effects ofmacroscopic mechanical drift and dust are minimized. See, for example,FIG. 1 where the first and second resonators 104 a and 104 b areinterwoven in plasmonic resonance detector 104, and FIG. 6 where thecarbon nanotube ribbons 602 a and 602 b (i.e., the resonant detectors)are interwoven in plasmonic resonance detector 600. Second, the resonantdetectors are more efficient at harvesting the energy at theirabsorption peak, because the presence of resonance enhances theabsorption of light. In turn, this efficiency allows the light source tobe less bright, and less power to be consumed by the overall detector.Third, additional wavelengths can easily be added to the sensor, forsensing other gases like carbon monoxide. Finally, the simpler design ofthe overall sensor, with an integrated filter and detector, can lead tomore flexible opportunities for deployment.

FIG. 7 is a diagram illustrating an exemplary gas sensor 700 employingcarbon nanotube plasmonic resonance detector 600 (of FIG. 6). In thisexample, the same set-up is used as in FIG. 1, except that the plasmonicresonance detector array 104 is configured as described in conjunctionwith the description of FIG. 6 above, i.e., having resonant detectorsformed from ribbons of carbon nanotubes of differing lengths. Likestructures are numbered alike in the figures. Arrow 702 is used toindicate the optical path between the light source 102 and the plasmonicresonance detector 600.

An exemplary methodology 800 for forming plasmonic resonance detector600 (of FIG. 6) is now described by way of reference to FIG. 8. In step802, thick films of aligned carbon nanotubes 820 are deposited onto afilter membrane 822 using a vacuum filtration technique. See, forexample, Chiu and Supporting Information for Chiu et al. “Strong andBroadly Tunable Plasmon Resonances in Thick Films of Aligned CarbonNanotubes,” Nano Letters, 2017, 17 (August 2017) (6 total pages)(hereinafter “Supporting Information for Chiu”), the contents of both ofwhich are incorporated by reference as if fully set forth herein. Forinstance, single-walled carbon nanotubes are first dispersed in anaqueous solution with a surfactant, e.g., sodium dodecylbenesulfonate(SDBS), and the dispersion is slowly vacuum-filtered onto a filtrationmembrane such as a polycarbonate filter membrane. If the filtrationspeed and surfactant concentration is optimized, this process results inaligned films of carbon nanotubes. See Chiu and Supporting Informationfor Chiu.

In step 804, the aligned carbon nanotubes 820 are then transferred fromthe filter membrane 822 to an insulating substrate 824. According to anexemplary embodiment, the insulating substrate 824 includes, but is notlimited to, high-resistivity silicon (Si), plastic, and glass. Inanother exemplary embodiment, the insulating substrate 824 includes aninsulating layer (on which the detector is formed) over a semiconductor.For instance, by way of example only, the insulating substrate 824 caninclude a bulk semiconductor (e.g., Si, germanium (Ge), silicongermanium (SiGe), III-V, etc.) wafer covered with a dielectric such assilicon dioxide (SiO₂), hafnium oxide (HfO₂) and/or aluminum oxide(Al₂O₃). Use of a semiconductor under the insulator enables theimplementation of electronics in the semiconductor (e.g., detectorreadout electronics and signal processing) that support the detector.

According to an exemplary embodiment, the aligned carbon nanotubes 820are transferred to the semiconductor substrate 824 by etching the filtermembrane 822. See, for example, Chiu and Supporting Information forChiu. Namely, the filter membrane 822 is placed face down on thesubstrate 824 with the aligned carbon nanotubes 820 in direct contactwith the dielectric. Pressing the filter membrane 822 onto the substrate824 (e.g., via a glass slide placed over the filter membrane 822) can beused to adhere the aligned carbon nanotubes 820 onto the substrate 824.An organic solvent is then used to dissolve the filter membrane 822,leaving behind the aligned carbon nanotubes 820 on the substrate 824 asshown in step 804. Suitable organic solvents include, but are notlimited to, N-methyl-2-pyrrolidone and/or chloroform.

In step 806, a patterned mask 826 is then formed over the aligned carbonnanotubes 820. The mask 826 can include a standard hardmask materialsuch as silicon nitride (SiN) patterned using standard opticallithography and etching techniques or, as described in Chiu, apoly(methyl methacrylate)/hydrogen silsesquioxane (PMMA/HSQ) maskpatterned using electron beam (e-beam) lithography. The mask 826 marksthe footprint and location of the ribbons of (horizontally-aligned)carbon nanotubes having different lengths. According to an exemplaryembodiment, the mask 826 defines carbon nanotubes having at least twolengths, L1 and L2—see above. By way of example only, as describedabove, L1 can correspond to the 4.3 μm CO₂ absorption and L2 to the 4.0μm reference.

In step 808, an etch is used to transfer the mask 826 to the alignedcarbon nanotubes 820, patterning the film of aligned carbon nanotubes820 into ribbons of carbon nanotubes having at least two differentlengths. For instance, in the example shown in FIG. 8, the alignedcarbon nanotubes 820 are patterned into ribbons 602 a and 602 b ofcarbon nanotubes having two lengths L1 and L2, wherein L1>L2. By way ofexample only, as provided above, L1 can correspond to the 4.3 μm CO₂absorption and L2 to the 4.0 μm reference. Suitable etching processesinclude, but are not limited to, reactive ion etching (RIE) in a plasmasuch as oxygen plasma. Following the nanoribbon etch, any remaining mask826 can be removed.

Finally, in step 810 metal contacts 604 and 606 are formed to theribbons 602 a and 602 b of carbon nanotubes, respectively. Standardmetallization techniques can be employed. Suitable contact metalsinclude, but are not limited to, copper (Cu), nickel (Ni), platinum (Pt)and/or palladium (Pd).

An exemplary methodology 900 for analyzing a target gas using thepresent plasmonic resonance gas sensors is now described by way ofreference to FIG. 9. The description of methodology 900 will use thedesign of gas sensor 100 (of FIG. 1) as a general reference point and,where appropriate, incorporate features of the various other embodimentsdescribed herein.

In step 902, a light source (e.g., light source 102) is used toilluminate the target gas within the enclosure 101. According to anexemplary embodiment, the light source is an incandescent light sourcethat is incident on the plasmonic resonance detector 104 as in FIG. 1 orthe plasmonic resonance detector 600 as in FIG. 6. In that case, gaswithin the enclosure 101 will cross the optical path between the lightsource and the plasmonic resonance detector. Alternative light sourcesare also described herein. For instance, a plasmonic light source isdescribed in conjunction with the description of FIG. 10 below, whichgenerates quasi-coherent light.

The target gas absorbs a specific wavelength of infrared light from thelight source. By way of example only, as provided above, CO₂ has anabsorption peak at 4.3 μm. As also provided above, the plasmonicresonance detector includes at least two different resonators, oneresonant at the absorption peak of the target gas and the other resonantat a reference wavelength (e.g., a 4.0 μm reference for 4.3 μm CO₂absorption). See, for example, first regions 104 a and second regions104 b of plasmonic resonance detector 104. For instance, in the case ofa carbon nanotube-based plasmonic resonance detector such as detector600 in FIG. 6, the carbon nanotubes are etched to at least two differentlengths (e.g., L1 and L2) corresponding to different plasmon resonancesin the carbon nanotubes, one at the absorption peak of the target gasand the other at a reference wavelength. Embodiments will also beprovided below with plasmonic resonance detectors that employ differentsized non-carbon nanotube resonators.

The notion here is that the resonators are configured (e.g., by theirsize/length) to resonate either at the same wavelength as the target gasor at a reference wavelength. Light from the light source will beabsorbed by the resonators, causing the resonators to be heated andthereby generate photocurrent signals I₁ and I₂ (see above) viaphotothermoelectric effects. When the target gas is present in theenclosure 101 in between the light source and the plasmonic resonancedetector, the target gas will also absorb that same wavelength ofinfrared light from the light source, with the amount of absorptionbeing proportional to the concentration of the target gas. Thus, theamount of light of that target wavelength incident on the resonatorswill be based on how much has already been absorbed by the target gas.The reference signal remains unchanged by the target gas. As such, thedifference in the photocurrent signals I₁ and I₂ (e.g., target gas andreference signals, respectively), i.e., I₁-I₂, will indicate an amountof the target gas in the enclosure 101. Further, a reading of I₁ and I₂without the target gas present can be used to establish a baselinereading of I₁-I₂ for comparison.

Namely, in step 904 the selective absorption of light by the (differentsized) resonators (e.g., L1 and L2 in the case of carbon nanotube-basedplasmonic resonance detector 600 of FIG. 6) causes the resonators to beheated to different temperatures, for example, a temp1 and a temp2. Asprovided above, the amount of light at the target wavelength incident onthe resonators is dependent on an amount of the light at the targetwavelength already absorbed by the target gas. Due to the Seebeckeffect, in step 906, a voltage is induced across the resonators,generating a photocurrent (I₁ or I₂) via photothermoelectrictransduction. A description of the strong thermoelectric effect incarbon nanotubes is provided in J. Hone et al., “Thermoelectric Power ofSingle-Walled Carbon Nanotubes,” Phys. Rev. Lett. 80, pgs. 1042-1045(February 1998), the contents of which are incorporated by reference asif fully set forth herein. In step 908, the signal I₁-I₂ is used todetermine the concentration of the target gas in the enclosure.

To use the above-described example of CO₂ detection to furtherillustrate this concept, assume that the resonators are configured toresonate at 4.3 μm (corresponding to the peak absorption of the targetgas CO₂) and 4.0 μm (as a reference). Without any CO₂ the signal fromthese resonators, i.e., I_(4.3)-I_(4.0) will have a certain value x.However, when CO₂ is present in the enclosure 101 in the light pathbetween the light source and the plasmonic resonance detector, itsabsorption at 4.3 μm will decrease an amount of infrared light at 4.3 μmincident upon the plasmonic resonance detector, thereby changing thephotocurrent signal I_(4.3) and output value x. The amount by whichI_(4.3) is changed is proportional to the concentration of CO₂ in theenclosure 101 (since a larger concentration of the CO₂ absorbs morelight at 4.3 μm, and vice versa). Namely, the amount by which outputvalue x changes is proportional to the concentration of the target gasCO₂.

As highlighted above, light sources other than an incandescent lightbulb are also contemplated herein. For instance, according to analternative embodiment, a plasmonic light source is employed in thepresent gas sensor design. See, for example, FIG. 10. As shown in FIG.10 plasmonic light source 1000 includes a plurality of plasmonicresonators (in this case aligned carbon nanotubes 1002) in between(first and second) electrodes 1004 and 1006. As shown in FIG. 10, avoltage source (labeled “Voltage”) applies a voltage across theelectrodes 1004 and 1006 causing current to pass through the plasmonicresonators, i.e., carbon nanotubes 1002. This causes the carbonnanotubes 1002 to heat up and emit quasi-coherent light via thermal(“blackbody”) radiation. In this case, the wavelength of the emittedlight will be modified from the blackbody spectrum via the Purcelleffect to be concentrated at the center wavelength of the resonators ofthe light source.

FIG. 11 is a diagram illustrating an exemplary gas sensor 1100 employingcarbon nanotube plasmonic resonance detector 600 (of FIG. 6) andplasmonic light source 1000 (of FIG. 10). In this example, the samegeneral set-up is used as in FIG. 1, i.e., the light source andplasmonic resonance detector being present within an enclosure 101 withthe light source being incident on the plasmonic resonance detector.Thus, like structures are numbered alike in the figures.

Non-carbon nanotube-based plasmonic resonance differential detectordesigns are also contemplated herein. See, for example, plasmonicresonance detector 1200 in FIG. 12. As shown in FIG. 12, plasmonicresonance detector 1200 includes a differential plasmon resonator array,where the resonators, i.e., (first and second sets of) plasmonicresonators 1202 a and 1202 b are split-ring resonators (instead ofcarbon nanotubes) of different sizes. Leveraging the same generalprinciples described above, based on their different sizes (e.g.,diameters D1 and D2, respectively) the split-ring resonators 1202 a and1202 b are configured to be resonant with light at differentwavelengths—thereby providing a differential signal. According to anexemplary embodiment, the first set of split-ring resonators is resonantat the absorption peak of a target gas, and the second set of split-ringresonators provides a differential signal at a slightly differentwavelength, or vice versa. For instance, by way of example only, thelarger resonators (split-ring resonators 1202 a of diameter D1) can havea resonance at 4.3 μm, at the CO₂ absorption band, and the smallerresonators (split-ring resonators 1202 b of diameter D2) can have aresonance at 4.0 μm (a reference wavelength), providing the differentialsignal.

With the split-ring resonators, the resonant wavelength is approximatelyproportional to the diameter (i.e., D1, D2, etc.) of the resonator. Thewidth of the split-ring resonator (i.e., outer diameter minus innerdiameter) and the out-of-plane height would be chosen so that theresonators effectively confine light, and the diameter would be chosenso that the appropriate wavelength resonator is obtained.

As shown in FIG. 12, (first and second) metal contacts 1204 and 1206 areformed to the split-ring resonators 1202 a and 1202 b, respectively.Specifically, the metal contacts 1204 are formed contacting thesplit-ring resonators 1202 a (having size S1), and the metal contacts1206 are formed contacting the split-ring resonators 1202 b (having sizeS2). The metal contacts 1204 and 1206 are used to extract the target gasabsorption signal (I₁′) and the reference signal (I₂′) from thesplit-ring resonators 1202 a and 1202 b, respectively, viaphotothermoeletric transduction (see above). For instance, in thenon-limiting example provided above, the metal contacts 1204 are used toextract the CO₂ absorption signal (I_(4.3)), and the metal contacts 1206are used to extract the reference signal (I_(4.0)).

According to an exemplary embodiment, one contact to each split-ringresonator is grounded while the other is connected (through wirebonds,chip carriers, etc.—not shown) to a voltage preamplifier, which willconvert the photovoltage to a macroscopic voltage that can be outputted.Specifically, as shown in FIG. 12, metal contacts 1204 to split-ringresonators 1202 a (i.e., the target gas signal resonators) include afirst contact 1204 a which is grounded and a second contact 1204 b thatis connected to a voltage preamplifier 1208. Likewise, metal contacts1206 to split-ring resonators 1202 b (i.e., the reference signalresonators) include a first contact 1206 a which is grounded and asecond contact 1206 b that is connected to a voltage preamplifier 1210.

An exemplary methodology 1300 for forming plasmonic resonance detector1200 (of FIG. 12) is now described by way of reference to FIG. 13. Instep 1302, a thin film 1320 (e.g., having a thickness t of from about 30nm to about 500 nm and ranges therebetween) of the resonator material isdeposited onto an insulating substrate 1322. Step 1302 is depicted via across-sectional side view in FIG. 13. Suitable resonator materialsinclude, but are not limited to, gold (Au), titanium nitride (TiN)and/or graphene. According to an exemplary embodiment, the insulatingsubstrate 1322 includes, but is not limited to, high-resistivity Si,plastic, and glass. In another exemplary embodiment, the insulatingsubstrate 1322 includes an insulating layer (on which the detector isformed) over a semiconductor. For instance, by way of example only, theinsulating substrate 1322 can include a bulk semiconductor (e.g., Si,Ge, SiGe, III-V, etc.) wafer covered with a dielectric such as SiO₂,HfO₂ and/or Al₂O₃. As provided above, use of a semiconductor under theinsulator enables the implementation of electronics in the semiconductor(e.g., detector readout electronics and signal processing) that supportthe detector.

Switching to a top-down view, in step 1304 a patterned mask 1324 is thenformed on the thin film 1320 marking the footprint and location of thesplit-ring resonators having different sizes/diameters. According to anexemplary embodiment, the mask 1324 defines split-ring resonators havingat least two diameters, D1 and D2—see above. By way of example only, asdescribed above, D1 can correspond to the 4.3 μm CO₂ absorption and D2to the 4.0 μm reference. The mask 1324 can include a standard hardmaskmaterial such as SiN patterned using standard optical lithography andetching techniques or a PMMA/HSQ mask patterned using e-beamlithography.

In step 1306, an etch is used to transfer the mask 1324 to the thin film1320, forming the split-ring resonators having at least two diameters.For instance, in the example shown in FIG. 13, the thin film 1320 ispatterned into split-ring resonators 1202 a and 1202 b having twodiameters D1 and D2, wherein D1>D2. By way of example only, as providedabove, D1 can correspond to the 4.3 μm CO₂ absorption and D2 to the 4.0μm reference. Suitable etching processes include, but are not limitedto, RIE in a plasma such as oxygen plasma. Following the resonator etch,any remaining mask 1324 can be removed.

Finally, in step 1308 metal contacts 1204 and 1206 are formed to thesplit-ring resonators 1202 a and 1202 b, respectively. Standardmetallization techniques can be employed. Suitable contact metalsinclude, but are not limited to, Cu, Ni, Pt and/or Pd.

In the examples above, reference was made to differential plasmonresonator arrays having resonators corresponding to the peak absorptionwavelength of a target gas and a reference wavelength. The same conceptscan be leveraged to extend the capabilities of the present gas sensorsto detect multiple target gases (i.e., a first target gas, a secondtarget gas, etc.) simply by expanding the array to include additional(interwoven) resonators. By way of example only, the plasmon resonatorarray 1400 shown in FIG. 14 includes ribbons 1402 a, 1402 b and 1402 cof carbon nanotubes having three different lengths, e.g., lengths L1′,L2′ and L3′-respectively, whereby the ribbons 1402 a of carbon nanotubeshaving the first length L1′ are resonant at a first wavelength, theribbons 1402 b of carbon nanotubes having the second length L2′ areresonant at a second wavelength, and the ribbons 1402 c of carbonnanotubes having the third length L3′ are resonant at a thirdwavelength. By way of example only, the lengths L1′, L2′ and L3′ mightcorrespond to the absorption peaks of a first target gas, a referencewavelength and a second target gas, generating photocurrent signals I₁″,I₂″ and I₃″, respectively.

Although illustrative embodiments of the present invention have beendescribed herein, it is to be understood that the invention is notlimited to those precise embodiments, and that various other changes andmodifications may be made by one skilled in the art without departingfrom the scope of the invention.

What is claimed is:
 1. A gas sensor, comprising: a plasmonic resonancedetector comprising a differential plasmon resonator array that isresonant at different wavelengths of light; and a light source incidenton the plasmonic resonance detector, wherein the light source comprisesan incandescent bulb.
 2. The gas sensor of claim 1, further comprising:an enclosure, wherein the plasmonic resonance detector and the lightsource are housed within the enclosure.
 3. The gas sensor of claim 1,wherein the enclosure comprises: an air inlet; and an air outlet.
 4. Thegas sensor of claim 1, wherein the differential plasmon resonator arraycomprises: at least one first set of plasmonic resonators interwovenwith at least one second set of plasmonic resonators, wherein the atleast one first set of plasmonic resonators is configured to be resonantwith light at a first wavelength, and wherein the at least one secondset of plasmonic resonators is configured to be resonant with light at asecond wavelength.
 5. The gas sensor of claim 4, wherein thedifferential plasmon resonator array further comprises: at least onethird set of resonators interwoven with the at least one first set ofplasmonic resonators and the at least one second set of plasmonicresonators, wherein the at least one third set of plasmonic resonatorsis configured to be resonant with light at a third wavelength.
 6. Thegas sensor of claim 5, wherein the first wavelength corresponds to apeak absorption wavelength of a first target gas, and wherein the secondwavelength corresponds to a reference wavelength.
 7. The gas sensor ofclaim 6, wherein the third wavelength corresponds to a peak absorptionwavelength of a second target gas.
 8. The gas sensor of claim 4, whereinthe differential plasmon resonator array comprises: a first set ofribbons of carbon nanotubes having a length L1 interwoven with a secondset of ribbons of carbon nanotubes having a length L2, wherein thecarbon nanotubes of the length L1 are configured to be resonant withlight at the first wavelength, and wherein the carbon nanotubes of thelength L2 are configured to be resonant with light at the secondwavelength.
 9. The gas sensor of claim 8, further comprising: firstmetal contacts to the first set of ribbons of carbon nanotubes havingthe length L1; and second metal contacts to the second set of ribbons ofcarbon nanotubes having the length L2.
 10. The gas sensor of claim 4,wherein the differential plasmon resonator array comprises: a first setof split-ring resonators having a diameter D1 interwoven with a secondset of split-ring resonators having a diameter D2, wherein thesplit-ring resonators having the diameter D1 are configured to beresonant with light at the first wavelength, and wherein the split-ringresonators having the diameter D2 are configured to be resonant withlight at the second wavelength.
 11. The gas sensor of claim 10, whereinthe first set of split-ring resonators and the second set of split-ringresonators are each formed from a material selected from the groupconsisting of: gold (Au), titanium nitride (TiN), graphene, andcombinations thereof.
 12. The gas sensor of claim 10, wherein thedifferential plasmon resonator array further comprises: contacts to thefirst set of split-ring resonators and second set of split-ringresonators.
 13. A method for analyzing a target gas, comprising thesteps of: illuminating the target gas with light from a light source,wherein the light is incident on a plasmonic resonance detectorcomprising a differential plasmon resonator array having at least onefirst set of plasmonic resonators interwoven with at least one secondset of plasmonic resonators, wherein the at least one first set ofplasmonic resonators is configured to be resonant with the light at afirst wavelength corresponding to a peak absorption wavelength of thetarget gas, wherein the at least one second set of plasmonic resonatorsis configured to be resonant with the light at a second wavelengthcorresponding to a reference wavelength, wherein the differentialplasmon resonator array comprises a first set of split-ring resonatorshaving a diameter D1 interwoven with a second set of split-ringresonators having a diameter D2, wherein the split-ring resonatorshaving the diameter D1 are configured to be resonant with the light atthe first wavelength, and wherein the split-ring resonators having thediameter D2 are configured to be resonant with the light at the secondwavelength; absorbing the light by the at least one first set ofplasmonic resonators and the at least one second set of plasmonicresonators, wherein the light at the first wavelength incident on the atleast one first set of plasmonic resonators is dependent on an amount ofthe light at the first wavelength absorbed by the target gas; generatinga photocurrent signal I₁ in the at least one first set of plasmonicresonators and a photocurrent signal I₂ in the at least one second setof plasmonic resonators; and determining a concentration of the targetgas using I₁-I₂.
 14. The method of claim 13, wherein the first set ofsplit-ring resonators and the second set of split-ring resonators areeach formed from a material selected from the group consisting of: Au,TiN, graphene, and combinations thereof.
 15. The method of claim 13,wherein the differential plasmon resonator array further comprises:contacts to the first set of split-ring resonators and second set ofsplit-ring resonators.
 16. A method for forming a plasmonic resonancedetector, comprising the step of: forming at least one first set ofplasmonic resonators interwoven with at least one second set ofplasmonic resonators, wherein the at least one first set of plasmonicresonators is configured to be resonant with light at a first wavelengthcorresponding to a peak absorption wavelength of a target gas, andwherein the at least one second set of plasmonic resonators isconfigured to be resonant with light at a second wavelengthcorresponding to a reference wavelength, wherein the method furthercomprises the steps of: forming a film of a resonator material on asubstrate, wherein the resonator material is selected from the groupconsisting of: Au, TiN, graphene, and combinations thereof; patterningthe film of the resonator material into a first set of split-ringresonators having a diameter D1 interwoven with a second set ofsplit-ring resonators having a diameter D2, wherein the split-ringresonators having the diameter D1 are configured to be resonant withlight at the first wavelength, and wherein the split-ring resonatorshaving the diameter D2 are configured to be resonant with light at thesecond wavelength; and forming contacts to the first set of split-ringresonators and second set of split-ring resonators.
 17. The method ofclaim 16, wherein the film of the resonator material has a thickness tof from about 30 nm to about 500 nm and ranges therebetween.
 18. Themethod of claim 16, wherein the substrate comprises an insulatingsubstrate.
 19. The method of claim 18, wherein the insulating substratecomprises a bulk semiconductor wafer covered with a dielectric.
 20. Themethod of claim 19, wherein the bulk semiconductor wafer comprises amaterial selected from the group consisting of: silicon (Si), germanium(Ge), silicon germanium (SiGe), III-V, and combinations thereof, andwherein the dielectric comprises a material selected from the groupconsisting of: silicon dioxide (SiO₂), hafnium oxide (HfO₂) aluminumoxide (Al₂O₃), and combinations thereof.